Fix z.any missing object keys

[cyphercodes]

Fixes #5997.

z.any() accepts undefined, so absent object keys using z.any() should not be rejected by the missing-key guard before the schema result is handled. This marks z.any() as optional on input while preserving its output requiredness, with coverage for both JIT and jitless object parsing.

[pullfrog]

TL;DR — Restores 4.3.x behavior for z.object({ k: z.any() }).parse({}) by marking z.any() as optin: "optional", so the object missing-key guard at handlePropertyResult no longer rejects absent z.any() keys. Output requiredness is preserved (optout stays undefined), so existing z.infer<> shapes are unchanged.

Key changes

• Mark $ZodAny as optional on input — adds the literal optin: "optional" on $ZodAnyInternals and assigns inst._zod.optin = "optional" in the constructor, matching the pattern used by $ZodDefault / $ZodPrefault / $ZodCatch.
• Coverage for the regression — new assertions on z.any()._zod.optin/optout in the optionality test, and a JIT + jitless z.object({ value: z.any() }).parse({}) case in object absent keys require optin optional.

_{Summary ｜ 2 files ｜ 1 commit ｜ base: main ← fix-any-object-missing-key-5997}

Fix scope and a parallel case worth flagging

Before: z.object({ a: z.any() }).parse({}) threw invalid_type / expected nonoptional on 4.4.x.
After: Parses to {}, matching 4.3.x semantics; { a: undefined } still produces { a: undefined }.

The fix is minimal and targeted at the exact seam called out in the learnings note on optin/optout semantics: z.object consults el._zod.optin === "optional" before the entry's schema runs, and $ZodAny previously didn't set that flag. Setting only optin (not optout) is the right call — output type for anyRequired: z.any() stays any (not any | undefined-keyed-optional), keeping the existing inferred type for unknown/any keys test in object.test.ts green.

One scoping question: z.unknown() has the same shape — $ZodUnknownInternals also lacks optin: "optional", and z.object({ a: z.unknown() }).parse({}) exhibits the same regression class as #5997. This PR scopes to z.any() because that's what the issue reports, which is reasonable, but @colinhacks may want to fold z.unknown() in here or track it separately.

packages/zod/src/v4/core/schemas.ts · packages/zod/src/v4/classic/tests/optional.test.ts

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[pullfrog]

Consider applying the same change to $ZodUnknown (lines 1465-1481). z.unknown() accepts undefined and exhibits the same regression — z.object({ a: z.unknown() }).parse({}) throws expected nonoptional on 4.4.x for the same reason. Optional follow-up; the linked issue scopes only to z.any().